Trajectory storage apparatus and method for surgical navigation system

ABSTRACT

Apparatus and methods are disclosed for use within an image-guided surgical navigation system for the storage and measurement of trajectories for surgical instruments. An icon representing the real-time trajectory of a tracked instrument is overlaid on one or more pre-acquired images of the patient. At the surgeon&#39;s command, the navigation system can store multiple trajectories of the instrument and create a static icon representing each saved trajectory for display. The surgeon may also measure a planar angle between any two trajectories. The angle is computed in the plane of the image, and therefore will be computed separately for each image displayed. Furthermore, the surgeon has the option of computing and displaying the three-dimensional distance between two points defined by any two trajectories.

BACKGROUND OF THE INVENTION

1. Field of Invention

The present invention is directed generally to image-guided medicalprocedures, and more particularly, to systems and methods for thestorage and geometric measurement of instrument trajectories used inimage-based surgical guided navigation systems.

2. Description of the Related Art

Image based surgical techniques have been used with success in aidingphysicians for performing a wide variety of delicate surgicalprocedures. These procedures are typically used when the visualizationof a surgical tool could be obscured by a patient's anatomy, or when thesurgical tool is visible but the patient's anatomy may be difficult tovisualize in three dimensions. Such procedures include, for example,spinal implant placement, the alignment of broken bone fragments, andthe fixation of bone fractures. Prior art techniques to accuratelyposition a surgical instrument have included the use of x-ray images tolocalize its position. Through the repeated acquisition of x-ray imagesduring the procedure, real-time placement of the instrument relative tothe patient's anatomy can be displayed. More recently, virtualfluoroscopically-based surgical navigation systems have been employed totrack an instrument trajectory and superimpose its representation ontopre-acquired images without requiring x-rays to be repeatedly takenduring the actual surgical procedure.

In many situations, a surgeon would like to create a static visualreference using the real-time and generally instantaneous instrumenttrajectory displayed by the surgical navigation system as the instrumentprogresses in the general direction of a selected, desired path. Forexample, some procedures require the serial placement of severalimplants which must be placed in a precise relative geometry. Currently,the surgeon must reacquire a new set of images after each implant isplaced to properly determine the trajectory of the subsequent implant.This can be a time consuming process which increases the amount ofradiation exposure to the patient and operating room personnel.

Other situations may require the surgeon to make accurate geometricmeasurements of a patient's anatomy. For example, some surgicalprocedures require the precise removal of a specific amount of bonetaken in the shape of a wedge. In order to determine this amount, anangular measurement of the bone at the surgical site would assist inthis procedure. Another example would be in allowing the surgeon to makedistance measurement between bone implant sites to ensure proper implantplacement. In light of the foregoing, there is a need for the ability tosave surgical instrument trajectories and have the capability to performmeasurements thereon.

SUMMARY OF THE INVENTION

The present invention is directed generally to image guided medicalprocedures, and, particularly, to medical procedures involving thetracking of surgical instruments. More specifically, the presentinvention is directed to a device and method for storing instrumenttrajectories.

To achieve these objects and other advantages and in accordance with thepurposes of the invention, as embodied and broadly described herein, theinvention is directed to an apparatus and method for the storage oftrajectories and measurements which may be performed thereon for use inconjunction with image-guided surgical navigation systems.

In one aspect of the invention, an instrument trajectory is tracked inreal-time by a surgical navigation system. An icon representing thisreal-time trajectory is overlaid on one or more pre-acquired images ofthe patient. At the surgeon's command, the navigation system can storethe trajectory of the instrument and, if desired, create a static iconrepresenting the saved trajectory for display on each pre-acquiredimage. The icon representing the stored trajectory is simultaneouslydisplayed with the real-time trajectory's icon so the surgeon mayvisually compare them. The surgeon has the option of saving additionaltrajectories by reissuing the storage command.

In another aspect of the invention, the surgeon may measure anglesbetween pairs of any two trajectories. The angles are computed in theplane of the image, and are, therefore, computed separately for eachimage displayed. One option is to compute one or more angles between thereal-time trajectory and saved trajectories. These angles are preferablycomputed and displayed on each pre-acquired image. As the real-timetrajectory changes, the displayed values are preferably updated in eachimage in real-time. Another option is to measure one or more anglesbetween pairs of any two stored trajectories. As with the prior option,these angles could be computed and displayed separately for each image.

In yet another aspect of the invention, three dimensional distancesbetween pairs of points defined by one or more sets of two trajectoriescan be computed and displayed. One option is to measure the distancebetween the real-time trajectory and one or more saved trajectories.These measurements would be computed in real-time and updated on thedisplay as the real-time trajectory varies. Another option would becomputing and displaying distances between pairs of points defined byone or more sets of two user-selected stored trajectories. For either ofthese two options, the defined points may be represented by the tip ofeach trajectory as computed by the system, or may be defined by auser-selected extension projected from the trajectory's tip.

Preferably, the invention can overcome the problems of the prior art byproviding the surgeon with the visual reference and measurementinformation required for some surgical procedures.

Both the foregoing general description and the following detaileddescription are exemplary, and are intended to provide furtherexplanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate an embodiment of the inventionand together with the description, serve to explain the principles ofthe invention.

FIG. 1 is a simplified block diagram of an embodiment of a system forthe storage and measurement of instrument trajectories in accordancewith the present invention.

FIG. 2 is a simplified side view of an embodiment of a system for use ininstrument trajectory storage and measurement in accordance with thepresent invention.

FIG. 3 is a block diagram of a process used to select, store, andcompute geometric properties of trajectories in accordance with thepresent invention.

FIG. 4 is an exemplary diagram of a display in accordance with anembodiment of the invention showing several stored instrumenttrajectories and the real-time trajectory superimposed on two images ofa patient's anatomy.

FIG. 5 is a simplified block diagram of an exemplary computer systemused in the surgical navigation system in accordance with one embodimentof the invention.

FIG. 6 is a block diagram of a process used compute the angle betweentwo trajectories in accordance with the present invention.

FIG. 7 is a is a block diagram of a process used compute the distancebetween the tips of two trajectories in accordance with the presentinvention.

DETAILED DESCRIPTION

Reference will now be made in detail to the present preferredembodiments of the invention, examples of which are illustrated in theaccompanying drawings. Wherever possible, the same reference numberswill be used throughout the drawings to refer to the same or like parts.

With reference to FIG. 1, there is shown schematically an apparatus inaccordance with the present invention for the storage of instrumenttrajectories. Image-based surgical navigation system 100 enables asurgeon to generate and display on monitor 115 the trajectory ofinstrument 125, which is preferably a surgical instrument configured inany known manner. Data representing one or more pre-acquired images 105is fed to navigation computer 110. Navigation computer 110 tracks theposition of instrument 125 in real-time utilizing detector 120. Computer110 then registers and displays the trajectory of instrument 125 withimages 105 in real-time. An icon representing the trajectory ofinstrument 125 is superimposed on the pre-acquired images 105 and shownon monitor 115. At the surgeon's command, the real-time trajectory ofinstrument 125 can be stored in computer 110. This command also createsa new static icon representing the trajectory of the instrument ondisplay 115 at the time the surgeon's command was issued. The surgeonhas the option of issuing additional commands, each one storing anreal-time trajectory and creating a new static icon for display bydefault. The surgeon can override this default and choose to not displayany static icon. The surgeon also has the option to perform a number ofgeometric measurements using the real-time and stored instrumenttrajectories. While the present invention described in more detail belowis exemplified by a fluoroscopic-based system, it is not limited to thedescribed embodiment and could be practiced with many different types ofnavigation systems.

FIG. 2 illustrates fluoroscopic image-based surgical navigation system200 according to the preferred embodiment of the present invention.System 200, described below in sufficient detail to allow anunderstanding and appreciation of the present invention, is explained ingreater detail in U.S. patent application Ser. No. 09/274,972 of DavidA. Simon et al., entitled “Navigation Guidance via Computer AssistedFluoroscopic Imaging,” filed on Mar. 23, 1999, the entire disclosure ofwhich is hereby incorporated by reference. However, it should beunderstood that the invention is not confined to use with thisparticular image guided surgical system.

Further referring to FIG. 2, an image-based surgical navigation system200 for acquiring and displaying x-ray images appropriate for a givensurgical procedure is shown. Pre-acquired images of patient 202 arecollected when a patient, lying on platform 205, is placed within C-arm212 of imaging device 210. The term “pre-acquired,” as used herein, doesnot imply any specified time sequence. Preferably, however, the imagesare taken at some time prior to when surgical navigation is performed.Usually, images are taken from two substantially orthogonal directions,such as anterior-posterior (A-P) and lateral, of the anatomy ofinterest. Imaging system 210 further includes x-ray source 214 and x-rayreceiving section 216 mounted on opposite sides of C-arm 212. Receivingsection 216 includes target tracking markers 222. System 210 furtherincludes C-arm control computer 226 which allows a physician to controlthe operation of imaging device 210. One implementation of imagingdevice 210 is the Model 9600 C-arm fluoroscope from OEC Medical Systems,Inc. of Salt Lake City, Utah, although tracking markers 222 aretypically not included in the Model 9600 C-arm fluoroscope and may haveto be added, however, the 9600 is otherwise structurally similar toimaging device 210. It is to be understood, however, that the inventionis not confined to the use of this type of imaging device.

Fluoroscopic images taken by imaging system 210 are transmitted tocomputer 226 where they may be forwarded to surgical navigation computer110. Computer 110 provides the ability to display the received imagesvia monitor 115. Other devices, for example, such as heads up displays,may also be used to display the images.

Further referring to FIG. 2, image-based surgical navigation system 100generally performs the real-time tracking of instrument 125, and, in thepreferred embodiment, also tracks the position of receiver section 216and reference frame 235. Detector 120 senses the presence of trackingmarkers on each object to be tracked. Detector 120 is coupled tocomputer 110 which is programmed with software modules that analyze thesignals transmitted by detector 120 to determine the position of eachobject in detector space. The manner in which the detector localizes theobject is known in the art, and is discussed, for example, in PCTApplication No. PCT/US95/12894 (Publication No. WO 96/11624) to Bucholz,the entire disclosure of which is incorporated by reference. Any type oftracking system known in the art can be used, including, by way ofexample only, acoustic, optical, LED/reflectors, electromagnetic, and/orother similar devices.

In general, instrument 125 is tracked by surgical navigation system 100using attached tracking markers 230 in order for its three-dimensionalposition to be determined in detector space. Computer 110 integratesthis information with the pre-acquired images of patient 202 to producea display which assists surgeon 270 when performing surgical procedures.An iconic representation of the trajectory of instrument 125 issimultaneously overlaid on the pre-acquired images of patient 202 anddisplayed on monitor 115. In this manner, surgeon 270 is able to see thetrajectory of the instrument relative to the patient's anatomy inreal-time.

Further referring to FIG. 2, the system according to the inventionpreferably has the ability to save the dynamic real-time trajectory ofinstrument 125. By issuing a command using foot-switch 280, for example,computer 110 receives a signal to store the real-time trajectory of theinstrument in the memory of computer 110. This “storage command” alsoinstructs computer 110 to generate a new static icon representing thesaved trajectory of the instrument, essentially “freezing” the icon atthe point when foot-switch 280 was closed. The static icon, along withthe icon representing the real-time trajectory of the instrument, can besimultaneously superimposed over the pre-acquired image. If multipleimages are being displayed, both static and real-time icons can besuperimposed on all of the displayed images. Other means of issuing thestorage command, such as, for example, through a graphical userinterface, may also be used. The surgeon also has the option of storingmultiple instrument trajectories. Each time a desired storage command isissued, the real-time trajectory of the instrument is stored in computer110 and a new static icon representing the stored trajectory isdisplayed on the pre-acquired image, or if more than one image is beingdisplayed, on all the pre-acquired images. These storage trajectoryfeatures are described in more detail below.

The system according to the invention preferably has the additionalcapability to measure angles between the real-time trajectory and one ormore of the stored trajectories. These “dynamic angles” are measured inthe image plane and are updated in real-time as the real-time trajectoryvaries. The computed values may then be displayed simultaneously withthe pre-acquired image. If more than one pre-acquired image is beingdisplayed, the angles for each image are preferably computed anddisplayed separately since they will be different for each image plane.Preferably, the system is configured to compute and display one or moreangles between pairs of stored trajectories selected by surgeon 270. Aswith the dynamic angle measurements, the angles between the storedtrajectories are computed in the image plane. They are preferablycalculated and displayed separately for each displayed image. Theseangle calculation features will be described in more detail below.

Furthermore, the system preferably also has the ability to computethree-dimensional distances between pairs of points defined by thereal-time trajectory and one or more stored trajectories selected bysurgeon 270. These “dynamic distance” values are displayed with theimage and vary as the instrument's trajectory changes. The system alsopreferably has the ability to measure distances between pairs of pointsdefined by one or more pairs of stored trajectories and to display thisinformation with the image. For either distance measuring option, thepoint pairs above may be defined by the tips of the instrumenttrajectories, or they may be defined by extending the tips by auser-specified amount. Each of these options will be discussed in moredetail below. Unlike the angle calculation, the three-dimensionaldistance is not a planar measurement, as such it will not vary amongdifferent images. The distance parameters may be displayed separatelyfor each image, or, as in the preferred embodiment, may only bedisplayed in one location.

Image-based surgical navigation system 100 utilized in the preferredembodiment of the present invention may be the same as that used in theFluoroNav™ system, which utilizes the StealthStation® Treatment GuidancePlatform, both of which are available from Medtronic Sofamor Danek, Inc.

FIG. 3 shows flowchart 300 illustrating the preferred embodiment forstoring instrument trajectories and computing geometric quantities.System 100 tracks instrument 125 by detecting tracking markers 230 withdetector 120. Positions are computed in real-time in detector space bycomputer 110 and converted to frame space, which is a coordinate systemassociated with reference frame 235. The conversions used may be oneswhich are well known to those skilled in the art. The instrumenttrajectory is preferably tracked using two points, the tip and the hind,on instrument 125 which are obtained using known offsets from trackingmarkers 230 (step 305). The computation of the tip and hind positions isdescribed in more detail below. An icon representing the real-timetrajectory of instrument 125 may be superimposed on one or morepre-acquired images 105 (step 310). The real-time instrument trackingproceeds until the computer receives a storage command from the surgeon.In the preferred embodiment, this command is given by a signal initiatedby actuation of foot-switch 280. The surgeon may also use a graphicaluser interface, which is described in more detail below, running oncomputer 110 to issue a storage command (step 315). Upon receipt of thecommand, computer 110 stores the real-time trajectory of instrument 125by placing the positions of the instrument's tip and hind into memory(step 320). Computer 110 then displays an icon representing the storedtrajectory which may be superimposed, along with the real-timetrajectory, on the pre-acquired image. If more than one pre-acquiredimage is being displayed, both the stored and real-time icons can besuperimposed on all pre-acquired images (step 325).

After one or more trajectories are saved, surgeon 270 has the option ofcomputing several geometric measurements through exercising theappropriate commands on the computer's 110 graphic interface (step 330).The surgeon will then typically select which trajectories to perform themeasurements upon. Measurements may be performed between the real-timetrajectory and one or more user-selected stored trajectories.Alternatively, a number of measurements may also be made between pairsof user-selected stored trajectories which are chosen through the userinterface (step 335). Once the trajectory pairs are chosen, surgeon 270can select to have the angles and/or a distance measurements performed(step 340). The distance and angle calculations are described below inmore detail.

Finally, the values the surgeon chose to measure can be displayed onmonitor 115 simultaneously with the pre-acquired image and trajectoryicons. If the measurement being performed includes the real-timetrajectory, each displayed value can be updated in real-time as theposition of instrument 125 changes. If multiple pre-acquired images arebeing displayed, the angles can be displayed on each desired image.However, in the preferred embodiment, the distance values will typicallybe displayed in one location. (step 345).

FIG. 4 shows an exemplary display which is consistent with the preferredembodiment of the invention. Display 400 preferably includes of threeprimary default windows, 403 a-c. Two fluoroscopic images, 402 and 405,taken from different orientations of a patient, are shown in twowindows, 403 a-b, respectively. Control and status window 403 c providesthe surgeon with a set of software controls to vary the systemparameters and control the modes and functionalities of the FluoroNav™system. Note that image 405 is partially obscured by dialog box 410.

Further referring to FIG. 4, image 402 shows seven stored trajectories,415 a-g, and one real-time trajectory, 417. The hind points of eachstored trajectory are denoted with cross symbols 420 a-f. Note the crosssymbol of trajectory 415 f is not indicative of a hind point, but of anextension. Extensions will be described in more detail later. The hindpoint of real-time trajectory 417 is indicated by cross 422. Obviously,many other symbols other than a cross may be used to denote the hindpoint. Each trajectory, represented by a directional indicator such as aline, can be automatically assigned a different color and uniquenumerical label to easily identify it. Other types of directionalindicators may also be used, and different shapes, styles, sizes, andtextures can be employed to differentiate among the trajectories. Onlylabels 425 a-d, associated with trajectories 415 a, 415 d, 415 f, and415 g, respectively, are shown in window 403 a. The surgeon has theoption of not showing the label for any trajectory if desired. Thesurgeon also has the option of changing the default color or label textfor any trajectory through the controls contained in dialog box 410. Thefunctionality of dialog box 410 will be described in more detail below.

In certain situations, the surgeon may wish to know where the tip of theinstrument would be if it were extended along a path direction indicatedby its current trajectory. When software button 430 in window 403 c istoggled on, computer 110 will calculate and display the icon based uponthe previously set extension, as set by slider bar 435, and the currenttrajectory of the instrument. Toggling button 430 again will result inno extension being displayed. For example, if button 430 were previouslyactivated and slider 435 is set to 45 mm, selecting button 430 will setthe slider value to 0 mm. Activating it a second time will restore it to45 mm. The estimated position of the tip can be calculated by computer110 by projecting a fixed length beyond the instrument's tip in thedirection of the line formed by each instrument's tip and hind. As shownin FIG. 4, an exemplary extension 418 is shown in a different line stylefrom trajectory icon 415 g. This difference could also be a change incolor, type, and/or texture between extension 418 and current trajectory415 g. Computer 110 may vary the length of the extension as directed bythe surgeon through manipulating control slider 435 using computer 110'smouse or keyboard. The extension feature is described in greater detailin U.S. patent application Ser. No. 09/274,972 which has beenincorporated by reference. Although the look-ahead technique describedabove projects the graphical representation of the instrument into theimage, there is no requirement that the instrument's graphicalrepresentation be in the space of the image for the extension to beprojected into the image. In other words, for example, the surgeon maybe holding instrument 125 above the patient and outside the space of theimage, so that the representation of the instrument does not appear inthe images. However, it may still be desirable to project ahead a fixedlength into the image to facilitate planning of the procedure.

Further referring to FIG. 4, dialog box 410 allows the surgeon tocontrol various aspects of how trajectories and labels are displayed.Whenever the surgeon initiates a command to store a trajectory, a row isautomatically created in dialog box 410 and is identified by a numberappearing in column 440. The surgeon has the option of removing a savedtrajectory from display 403 a by selecting the appropriate button incolumn 445. The color or texture of a button in column 445 can indicatethe current icon display status.

Column 450 contains fields which indicate the text used in the labelsfor each stored trajectory. Computer 110 can select numerical values asdefaults, which are illustrated in labels 425 a-d, or the surgeon mayselect a custom label. This is accomplished by using computer 110'smouse to select the appropriate field of column 450 corresponding to thestored trajectory to be renamed. Once selected, the surgeon can usecomputer 110's keyboard to enter the desired text for the label.Furthermore, the label of each trajectory can be selectively displayedby activating the appropriate button in column 455 with the mouse. Thecolor or texture of the button can be used to indicate the displaystatus of the label for each stored trajectory. In this example, buttonscorresponding to trajectories 1,5,6, and 7 are in the “on” state whichresults only in labels 425 a-d being displayed in window 403 a.

Selection of one of the buttons in column 456 causes the default colorof the stored trajectory to be overridden by the user. Activation of theappropriate button displays a palette of colors from which one maychoose to color the respective icon.

The surgeon also has the ability to select the mode of display for eachicon. Selecting pull-down menu 458 allows the user to chose from one ofthree different display modes for each stored trajectory. The firstmode, “Hind->Tip,” creates an icon by drawing a line from theinstruments hind position to the instruments tip position as shown inicons 415 a-e. The second mode, “Tip->Ext.,” creates an icon by drawinga line from the instrument's tip to the end of the extension. This modeis shown in icon 415 f, which is displayed as a light colored cross todenote the extension. The third display mode, “Hind->Ext.,” draws a linefrom the hind of the instrument to the tip of the extension. This modeis exemplified in icon 415 g and extension 418. Column 457 indicates thedisplay mode associated with each stored trajectory.

Further referring to FIG. 4, the surgeon has the option to have computer110 compute and display measurements between selected trajectories.Button 459 commands computer 110 to display the measurements and allowsthe user to select which measurements to display. Pull-down menus 461,463 allow the user to choose the trajectories which will be used toperform the measurements. Note that the real-time instrument may be usedin conjunction with one or more pairs of any of the stored trajectories,or the measurements may be made against one or more pairs of any twosaved trajectories. Text fields 460, 462 indicate which trajectorieswill be used in the measurement. The results of the measurementcalculation will be displayed in windows 403 a-b. In FIG. 4, the planarangle between real-time trajectory 417 and stored trajectory 415 a isshown at 470 in window 403b and 480 in window 403 a. The differencebetween values 470 and 480 is due to the different planar geometriesassociated with images 402 and 405.

Referring to FIG. 5, components and modules of a computer system 110used to perform various processes of the present invention aredescribed. Although a STEALTH STATION® image guided system manufacturedby Medtronic Sofamor Danek has been identified, it will be appreciatedthat the present invention may be utilized with other types of computersystems. One aspect of the computer system 110 includes a graphical userinterface system operating in conjunction with a display screen of adisplay monitor 115. The graphical user interface system is preferablyimplemented in conjunction with operating system 515 running computer110 for displaying and managing the display objects of the system. Thegraphical user interface is implemented as part of the computer system110 to receive input data and commands from a conventional keyboard 520and mouse 525. Foot-switch 280 is also configured to enable the user toinitiate the storage of instrument 125's real-time trajectory. Forsimplicity of the drawings and explanation, many components of aconventional computer system have not been illustrated such as addressbuffers, memory buffers, and other standard control circuits becausethese elements are well known in the art and a detailed descriptionthereof is not necessary for understanding the present invention.

A computer program used to implement the various steps of the presentinvention is generally located in memory unit 500, and the processes ofthe present invention are carried out through the use of a centralprocessing unit (CPU) 505. Those skilled in the art will appreciate thatthe memory unit 500 is representative of both read-only memory andrandom access memory. The memory unit also contains a database 550 thatstores data, for example, image data and tables, including suchinformation as stored instrument tip and hind positions, extensionvalues, and geometric transform parameters, used in conjunction with thepresent invention. CPU 505, in combination with the computer softwarecomprising operating system 515, scanning software module 530, trackingsoftware module 535, calibration software module 540, and displaysoftware module 545, controls the operations and processes of computersystem 110. The processes implemented by CPU 505 may be communicated aselectrical signals along bus 560 to an I/O interface 570 and a videointerface 575.

Scanning software module 530 performs the processes associated withcreating a coordinate reference system and reference images for use inconnection with the present invention and are known to those skilled inthe art. Tracking software module 535 performs the processes necessaryfor tracking objects in an image guided system as described herein andare known to those skilled in the art. Calibration software module 640computes the geometric transform which corrects for image distortionsand registers the images to the anatomical reference frame 235, and thusthe patient's anatomy.

Display software module 545 applies, and if desired, computes theoffsets between the guide tracking markers 230 and the tip and hind ofthe instrument in order generate an icon representing the trajectory ofthe instrument for superposition over the images. For instruments withfixed lengths and angulations, these offsets can be measured once andstored in database 550. The user would then select from a list ofinstruments, the one being used in the procedure so the proper offsetsare applied by display software module 545. For instruments withvariable lengths and angulations, the offsets could be measured manuallyand entered via keyboard 520, or measured using the navigation system100 in conjunction a tracked pointer or tracked registration jig (notshown). If a tracked pointer is used, the user will touch the tip andtail of the instrument while it is being tracked. The offsets arecomputed by display software module 545 and stored for later use.Similarly, if a tracked registration jig is used, the instrument isplaced within the jig while it is being tracked. The jig will measurethe extremities of the instrument and display software module 545 willagain compute the offsets and store them for later use in database 550.

Pre-acquired image data 105 can be fed directly into computer 110digitally through I/O interface 570, or may be supplied as video datathrough video interface 575. In addition, items shown as stored inmemory can also be stored, at least partially, on hard disk 580 ifmemory resources are limited. Furthermore, while not explicitly shown,image data may also be supplied over a network, through a mass storagedevice such as a hard drive, optical disks, tape drives, or any othertype of data transfer and storage devices which are known in the art.

FIG. 6 shows a block diagram illustrating a method for calculating aplanar angle between two trajectories for the preferred embodiment.After surgeon 270 selects the trajectories using pull-down menus 461,463as described above, the tip and the hind positions of each trajectoryare projected into the image plane. Since x-ray receiving section 216 istracked by navigation system 100 utilizing tracking markers 222, thecoordinates of the image plane are well defined. Using the tip, hind,and image plane coordinates, the projection is performed usingtechniques well known to those skilled in the art (610). A first linesegment is constructed by connecting the projected tip and hind pointscorresponding to the first trajectory computed above (step 620). In thesame manner, a second line segment is constructed utilizing theprojected points of the second trajectory (step 630). The anglecorresponding to the intersections of the two aforementioned linesegments can then be computed by techniques known to those skilled inthe art (step 640).

FIG. 7 shows a block diagram illustrating a method of the preferredembodiment for calculating a distance between points in space as definedby two trajectories. After surgeon 270 selects two trajectories usingpull-down menus 461, 463 in the manner described above, a first point isdetermined using the three-dimensional tip position of the firsttrajectory as computed by navigation system 100. If a “look-ahead”extension is associated with the first trajectory, it is added to thetip position (step 710). A second point associated with the secondtrajectory is computed in the same manner as described for step 710(step 720). The distance may then be computed using thethree-dimensional coordinates of the first and second points by astandard Euclidean distance formula which is well known in the art.

The foregoing description is presented for purposes of illustration andexplanation. It is not intended to be exhaustive or to limit theinvention to the precise form disclosed, and modifications of variationsare possible in light of the above teachings or may be acquired frompractice of the invention. The principles of the invention and itspractical application enable one skilled in the art to utilize theinvention in various embodiments and with various modifications as aresuited to the particular use contemplated.

For example, pre-acquired images obtained from different modalities maybe used in place of those produced by the C-arm fluoroscope x-rayimager. Such modalities include, by way of example only, computertomography, ultrasound, PET, or magnetic resonance imaging.

What is claimed:
 1. A method for storing a trajectory of an objectduring a surgical procedure using a surgical navigation system,comprising: providing at least one pre-acquired image of a patient;tracking a three-dimensional trajectory of the object; displaying afirst representation of the object's first trajectory superimposed ontothe at least one pre-acquired image; receiving a command to store theobject's first trajectory; storing the first representation of theobject's first trajectory; and superimposing a second representation ofthe object's second trajectory onto the at least one pre-acquired image.2. A method for storing a trajectory of an object during a surgicalprocedure using a surgical navigation system, comprising: providing atleast one pre-acquired image of a patient; tracking a three-dimensionaltrajectory of the object; displaying a first representation of theobject's first trajectory superimposed onto the at least onepre-acquired image; receiving a command to store the object's firsttrajectory; storing the first representation of the object's firsttrajectory; superimposing a second representation of the object's secondtrajectory onto the at least one pre-acquired image; receiving aplurality of commands wherein each of the plurality of commandsinitiates storage of the object's trajectory at a different time;storing the object's trajectory corresponding to said different times;and superimposing a plurality of representations of the object's storedtrajectories onto the at least one pre-acquired image.
 3. The method ofclaim 2 further including: employing directional indicators toillustrate the first representation and each of the plurality ofrepresentations, wherein each directional indicator has at least one ofa different color, style, shape, size, and texture.
 4. The method ofclaim 2 further including: automatically assigning at least onerepresentation with a unique label, said label being displayed adjacentto its corresponding representation.
 5. The method of claim 4 whereinsaid label includes at least one alpha-numeric character, the characterbeing one of generated automatically and specified manually.
 6. Themethod of claim 2 further including: selecting the first and secondtrajectory; projecting both a tip position and hind position of thefirst trajectory into a plane of the at least one pre-acquired image toform a first set of projected points; projecting both a tip position andhind position of the second trajectory into the plane of the at leastone pre-acquired image to form a second set of projected points;calculating a first segment connecting the first set of projectedpoints; calculating a second segment connecting the second set ofprojected points; computing an angle between said first and said secondsegments for each pre-acquired image; and displaying the computed anglewith its respective image.
 7. The method of claim 7 wherein the firstand second trajectories are one of real-time trajectories and storedtrajectories.
 8. The method of claim 7 wherein a plurality of trajectorypairs are used to compute and display a plurality of angles.
 9. A methodfor storing a trajectory of an object during a surgical procedure usinga surgical navigation system, comprising: providing at least onepre-acquired image of a patient; tracking a three-dimensional trajectoryof the object; displaying a first representation of the object's firsttrajectory superimposed onto the at least one pre-acquired image;receiving a command to store the object's first trajectory; storing thefirst representation of the object's first trajectory; superimposing asecond representation of the object's second trajectory onto the atleast one pre-acquired image; selecting the first and second trajectory;computing a first point based on adding a tip position of the firsttrajectory to an extension associated with the first trajectory;computing a second point based on adding a tip position of the secondtrajectory to an extension associated with the second trajectory;computing a distance in three-dimensional space between the first pointand the second point; and displaying the computed distancesimultaneously with each image.
 10. The method of claim 9 wherein thefirst and second trajectories are one of real-time trajectories andstored trajectories.
 11. The method of claim 10 wherein a plurality oftrajectory pairs are used to compute and display a plurality ofdistances.
 12. The method of claim 1 further comprising providing aplurality of pre-acquired images taken from different orientations andsimultaneously displaying more than one of the plurality of pre-acquiredimages.
 13. The method of claim 1 wherein the pre-acquired images areimages acquired by using a C-arm fluoroscope.
 14. The method of claim 1wherein the command is initiated through a software user-interface. 15.The method of claim 1 wherein the command is initiated by a foot switch.16. An apparatus for storing a trajectory of an object during a surgicalprocedure using a surgical navigation system, comprising: a computerprocessor; a sensor of a three-dimensional position of the objectoperatively connected to the computer processor; a memory coupled to thecomputer processor, storing: at least one pre-acquired image of apatient; instructions that when executed by the computer processor trackthe trajectory of the object, store the trajectory into memory uponreceiving a storage command, generate representations of the trackedtrajectory and the stored trajectory, and superimpose therepresentations onto the at least one pre-acquired image; and a displaycoupled to the processor for the superimposed representations to bedisplayed on the at least one pre-acquired image.
 17. The apparatus ofclaim 16 further comprising computer instructions that when executed bythe computer processor: store a plurality of tracked trajectories uponreceiving additional storage commands, at least one stored trajectoryrepresenting a corresponding one of the plurality of trackedtrajectories at the time the storage command is received; generate aplurality of representations for selected ones of the plurality ofstored trajectories; and superimpose the plurality of representations ofthe object's stored trajectories onto the at least one pre-acquiredimage.
 18. The apparatus of claim 17 wherein: directional indicators areused to illustrate the tracked representation and each of the pluralityof stored representations, wherein each directional indicator has atleast one of a different color, style, size, shape, and texture.
 19. Theapparatus of claim 17 further comprising: a plurality of unique labelsthat are automatically assigned by the computer processor to each storedrepresentation, each label being displayed adjacent to its correspondingrepresentation.
 20. The apparatus of claim 19 wherein each labelincludes at least one alpha-numeric character, the character being oneof generated automatically and specified manually.
 21. The apparatus ofclaim 17 further comprising computer instructions that when executed bythe computer processor: select a first and second trajectory frommemory; project both a tip position and hind position of the firsttrajectory into a plane of at least one pre-acquired image to form afirst set of projected points; project both a tip position and hindposition of the second trajectory into the plane of at least onepre-acquired image to form a second set of projected points; calculate afirst segment connecting the first set of projected points; calculate asecond segment connecting the second set of projected points; computethe angle between said first and said second segments for eachpre-acquired image; and display the computed angle simultaneously withits respective image on the display.
 22. The apparatus of claim 21wherein the first and second trajectories are one of trackedtrajectories and stored trajectories.
 23. The apparatus of claim 22wherein a plurality of first and second trajectory pairs are used tocompute and display a plurality of angles on the display.
 24. Anapparatus for storing a trajectory of an object during a surgicalprocedure using a surgical navigation system, comprising: a computerprocessor; a sensor of a three-dimensional position of the objectoperatively connected to the computer processor; a memory coupled to thecomputer processor, storing: at least one pre-acquired image of apatient; instructions that when executed by the computer processor trackthe trajectory of the object, store the trajectory into memory uponreceiving a storage command, generate representations of the trackedtrajectory and the stored trajectory, and superimpose therepresentations onto the at least one pre-acquired image and furtherinstructions that when executed by the computer processor: select afirst and second trajectory from memory; compute a first point based onadding a tip position of the first trajectory to an extension associatedwith the first trajectory; compute a second point based on adding a tipposition of the second trajectory to an extension associated with thesecond trajectory; compute a distance in three-dimensional space betweenthe first point and the second point; display the computed distancesimultaneously with each image on a display; and the display coupled tothe processor for the superimposed representations to be displayed onthe at least one pre-acquired image.
 25. The apparatus of claim 24wherein the first and second trajectories are one of trackedtrajectories and stored trajectories.
 26. An apparatus for storing atrajectory of an object during a surgical procedure using a surgicalnavigation system, comprising: a computer processor; a sensor of athree-dimensional position of the object operatively connected to thecomputer processor; a memory coupled to the computer processor, storing:at least one pre-acquired image of a patient; instructions that whenexecuted by the computer processor track the trajectory of the object,store the trajectory into memory upon receiving a storage command,generate representations of the tracked trajectory and the storedtrajectory, and superimpose the representations onto at least onepre-acquired image; and a display coupled to the processor for thesuperimposed representations to be displayed on the at least onepre-acquired image, wherein a plurality of first and second trajectorypairs are used to compute and display a plurality of distances betweenthe trajectory pairs.
 27. The apparatus of claim 16 further comprising aplurality of pre-acquired images taken from different orientations forsimultaneous display.
 28. The apparatus of claim 16 wherein thepre-acquired images are images acquired from a C-arm fluoroscope coupledto the computer processor.
 29. The apparatus of claim 16 wherein thestorage command is initiated through a software user-interface runningon the computer processor.
 30. The apparatus of claim 16 wherein thestorage command is initiated by a foot switch.
 31. A system for storingthe trajectory of an object during a surgical procedure, comprising:means for tracking a three-dimensional trajectory of the object; meansfor providing at least one pre-acquired image of a patient to thetracking means; means for displaying a first representation of theobject's first trajectory superimposed onto the at least onepre-acquired image; means for receiving a command to store the object'sfirst trajectory; means for storing the first representation of theobject's first trajectory; and means for superimposing a secondrepresentation of the object's second trajectory onto the at least onepre-acquired image.
 32. A system for storing the trajectory of an objectduring a surgical procedure, comprising: means for tracking athree-dimensional trajectory of the object; means for providing at leastone pre-acquired image of a patient to the tracking means; means fordisplaying a first representation of the object's first trajectorysuperimposed onto the at least one pre-acquired image; means forreceiving a command to store the object's first trajectory; means forstoring the first representation of the object's first trajectory; meansfor superimposing a second representation of the object's secondtrajectory onto the at least one pre-acquired image; means for receivinga plurality of commands wherein each of the plurality of commandsinitiates storage of the object's trajectory at a different time; meansfor storing the object's trajectory at different times; and means forsuperimposing a plurality of representations of the object's storedtrajectories onto the at least one pre-acquired image.
 33. The system ofclaim 32 further including: means for employing directional indicatorsto illustrate the first representation and each of the plurality ofrepresentations, wherein each directional indicator has at least one ofa different color, style, size, shape, and texture.
 34. The system ofclaim 32 further including: means for automatically assigning the firstrepresentation and each of the plurality of representations with aunique label, each label being displayed adjacent to its correspondingrepresentation.
 35. The system of claim 34 wherein each label includesat least one alpha-numeric character, the character being one ofgenerated automatically and specified manually.
 36. The system of claim32 further including: means for selecting the first and secondtrajectory; means for projecting both a tip position and hind positionof the first trajectory to form a first set of projected points; meansfor projecting both a tip position and hind position of the secondtrajectory to form a second set of projected points; means for forming afirst segment connecting the first set of projected points; means forforming a second segment connecting the second set of projected points;means for computing the angle between said first and said secondsegments for each pre-acquired image; and means for displaying thecomputed angle simultaneously with its respective image.
 37. The systemof claim 36 wherein the first and second trajectories are one ofreal-time trajectories and stored trajectories.
 38. The system of claim37 wherein a plurality of trajectory pairs are used to compute anddisplay a plurality of angles.
 39. A system for storing the trajectoryof an object during a surgical procedure, comprising: means for trackinga three-dimensional trajectory of the object; means for providing atleast one pre-acquired image of a patient to the tracking means; meansfor displaying a first representation of the object's first trajectorysuperimposed onto the at least one pre-acquired image; means forreceiving a command to store the object's first trajectory; means forstoring the first representation of the object's first trajectory; meansfor superimposing a second representation of the object's secondtrajectory onto the at least one pre-acquired image; means for selectingthe first and second trajectory; means for computing a first point basedon adding a tip position of the first trajectory to an extensionassociated with the first trajectory; means for computing a second pointbased on adding a tip position of the second trajectory to an extensionassociated with the second trajectory; means for computing a distance inthree-dimensional space between the first point and the second point;and means for displaying the computed distance simultaneously with eachimage.
 40. The system of claim 39 wherein the first and secondtrajectories are one of real-time trajectories and stored trajectories.41. The system of claim 40 wherein a plurality of first and secondtrajectory pairs are used to compute and display a plurality ofdistances.
 42. The system of claim 31 further comprising means forproviding a plurality of pre-acquired images taken from differentorientations and simultaneously displaying more than one of theplurality of pre-acquired images.
 43. The system of claim 31 wherein thepre-acquired images are images acquired by using a C-arm fluoroscope.44. The system of claim 31 wherein the command is initiated through asoftware user-interface.
 45. The system of claim 31 wherein the commandis initiated by a foot switch.
 46. A method for storing paths of anobject during a surgical procedure using a surgical navigation system,comprising: providing at least one pre-acquired image of a patient;tracking a three-dimensional path of the object; displaying a firstrepresentation of the object's path superimposed onto the at least onepre-acquired image; receiving at least one command to store the object'spath at different times; storing at least one object's pathcorresponding to said different times; superimposing at least onerepresentation of the object's stored path onto the at least onepre-acquired image; and computing at least one geometric measurementrelating to at least one of the object's path and at least one of thestored paths.